No, it's because as all of B&N's materials on the Nook and NookColor make clear, they can only be sold in the US. It isn't just your credit card that keeps your from buying -- although it does; your IP address also identifies you as being outside the US.
B&N probably doesn't believe that it can provide a sufficient support level outside the US, but B&N doesn't set the geographical limits on eBook sales -- those are set by the publishers.
You're probably out of luck with respect to apps, but you have plenty of options for buying eBooks. You can buy a wide range of eBooks here:
You can also find many free public domain books here:
*ETA* I can't buy from Saraiva or Cultura -- because they can't sell eBooks outside Brazil! So it goes both ways.
I recently bought a Nook Color in the US and I'm very satisfied with the product except by one point: I'm not able to buy any book or app from Brazil, were I live. I'd like to know if this is because my credit card is from outside the US of if this is happening because the nook is outside the country. I honestly don't see a point in keeping this restriction in times of so many offers in ereaders and tablets. I like very much my nook, and I believe you should consider opening your doors to other markets.
As others have said, it's generally a publisher geographic copyright restriction, not a policy B&N can change. Some authors have different publishers in different countries, therefore, the US publisher doesn't have the rights to sell the book in the other country, that has to be negotiated with the publisher for that country. B&N has never been an international company, so to sell books in foreign countries, they'd have to start from scratching negotiating all of the necessary contracts.
Thanks for the answers. I realize that I can buy books in other libreries, but I still can't solve the app issue. Hope I can buy some of them in my next trip to the US!